Yiddish arose in Central Europe. Nevertheless, the language includes a Semitic Component comprising thousands of lexical items that is synchronically fused with the Germanic Component within Yiddish. Theories from the sixteenth century to the present have contended that Semitisms entered a previously (nearly) wholly Germanic language from sacred Hebrew and. Aramaic texts used in the traditional Yiddish speaking civilization known as Ashkenaz. The thesis challenges the text theory. The alternative proposed is the continual transmission theory claiming that the Semitic Component entered Europe in the vernacular of the original settlers who were, retroactively speaking, the first Ashkenazim. Questions concerning the origin of the Semitic Component are also relevant to the determination of the relative age of Yiddish and to the contested status of the protolanguage within historical linguistics. The Semitic Component of all known Yiddish dialects is characterized by a system of long and short vowels in open syllables, reduced to a system of short vowels only in closed syllabic position. The resulting morphophonemic alternations and unique segmental distribution are shared neither by the Germanic Component nor the relevant varieties of traditional Hebrew and Aramaic. Nearly all nineteenth and twentieth century theories submit that an erstwhile system of five short vowels expanded in consequence of open syllable lengthening, a sound change triggered by the analogous German development. The standard theories are challenged by internal and comparative reconstruction as well as the results of transcomponent reconstruction, a method proposed for use with fusion languages such as Yiddish. Phonological proofs put forward, demonstrate that the Semitic Component entered Yiddish with its unique vocalism, including the later attested morphophonemic alternations, and can derive exclusively from a prelanguage. Moreover, Yiddish provides evidence for the recovery of a lost Northwest Semitic vowel system midway between the known Tiberian and Palestinian varieties.